This invention relates to an improved ignition device for an internal combustion engine, wherein output from a DC-DC converter connected to a battery charges a capacitor connected in series to the primary winding of an ignition coil, and electric energy stored in the capacitor is discharged through the primary winding in exact timing with the ignition of the internal combustion engine.
The so-called capacitor discharge type ignition device is the one wherein the stepped up voltage of a battery charges a capacitor connected in series to the primary winding of an ignition coil, and electric energy stored in the capacitor is discharged through the primary winding in synchronization with the ignition of the internal combustion engine to fire a gaseous mixture. The prior art capacitor discharge type ignition device is arranged, for example, as shown in FIG. 1. A DC-DC converter for stepping up the voltage of a battery 1 to high DC voltage comprises a circuit of resistors 3, 4 connected in series to the battery 1 through an ignition switch 2; a pair of oscillation transistors 5, 6; an oscillation voltage stepup transformer 7; a primary winding 8, a feedback winding 9 and secondary coil 10 constituting said transformer 7; and a rectification circuit consisting of diodes 11, 12, 13 and 14.
Positive output from the rectification circuit is supplied to one end of a resistor 15, the anode of an SCR 16 and one end of a capacitor 18. The capacitor is connected in series to the primary winding 20 of an ignition coil 19, the secondary winding 21 of which is connected to an ignition plug 22. The SCR 16 is connected in parallel with the primary winding 20.
Positive output from the battery 1 is delivered to a contact breaker 25 actuated by a cam 24 through the ignition switch 2 and resistor 23. A rectangular wave signal resulting from the intermittent actuation of the contact breaker 25 is differentiated by a differentiation circuit formed of a capacitor 26 and resistor 27. Positive differentiated pulses alone are impressed on the gate 17 of the SCR 16 through a diode 28.
When the ignition switch 2 is closed, the transistor 5, for example, is rendered conducting and the transistor 6 is kept in the turned off condition. Where current running through the transistor 5 is saturated at a prescribed level, voltage induced in the feedback winding 9 is reduced to zero, causing the transistor 5 to be turned off and the transistor 6 to be actuated. Where the transistor 6 is saturated, the transistor 5 is conversely rendered conducting. Thus, the above-mentioned oscillation circuit commences oscillation at a frequency of, for example, 8 KHz to 15 KHz. AC voltage impressed on the primary winding 8 is transformed to AC voltage of about 250 to 300 volts in the secondary coil 10 of the transformer 7. The latter AC voltage is rectified by the rectification circuit consisting of the diodes 11 to 14, and the resulting DC voltage is stored in the capacitor 18. On the other hand, the gate 17 of the SCR 16 is supplied with positive pulses generated when the contact breaker 25 is rendered OFF at the start of the internal combustion engine. As the result, the SCR 16 is rendered conducting, causing electric energy stored in the capacitor 18 to be rapidly discharged through the primary winding 20 of the ignition coil 19. Since the number of turns constituting the primary winding 20 and that of the secondary winding 21 of the ignition coil 19 generally bear the ratio of 1 : 100 to 1 : 150, as high voltage as 25000 to 45000 volts is impressed across the spark gap of the ignition plug 22, giving rise to discharges across said spark gap and in consequence igniting a gaseous mixture.
With the prior art ignition device arranged as described above, the discharge capacitor 18 is fully stored with rectification output from the DC-DC converter, where the internal combustion engine is driven at a relatively low speed, because one cycle of charge and discharge takes a long period. Where, however, said engine is driven at an increasing speed, then one cycle of charge and discharge is carried out in a shorter time, failing to attain the full charge of the capacitor 18. Namely, the SCR 16 is undesirably supplied with gating pulses while the capacitor 18 is not fully charged, resulting in the discharge of low electric energy. Accordingly, the discharge of an extremely small amount of electric energy fails to deliver full current to the ignition coil 19, leading to the supply of weak spark energy to the ignition plug 22 and in consequence the insufficient firing of a gaseous mixture.
To avoid the above-mentioned difficulties, it is necessary to store the discharge capacitor 18 with electric energy rapidly during the high speed drive of an internal combustion engine and, to this end, draw out high voltage output from the DC-DC converter. Where, however, high voltage output is generated by the DC-DC converter, then high voltage is also impressed between the anode and cathode of the SCR 16. Therefore, though expected to become nonconducting, the SCR 16 gets into operation under a very unstable condition, when impressed with voltage approaching a breakover level. For example, discharge is often commenced even when gating pulses are not supplied. Since output voltage from an alternator varies with the number of rotations of a driven internal combustion engine, the resultant change in the voltage at the terminal of the battery 1 charged with said alternator output affects output from the DC-DC converter. Thus any slight variation of voltage unnecessarily renders the SCR 16 conducting. The above-mentioned objectionable events take place independently of the application of gating pulses delivered to the SCR 16, leading to the disorderly rotation of the internal combustion engine. To this end, it may be considered to use the SCR which is capable of increasing high forward peak suppressing voltage. However, such type of an SCR is not preferred in respect of cost, manufacture and quality because it is expensive and bulky, has to be supplied with high input voltage for control of its gate, and makes a slow response to control.